1. Field of the Invention
This invention relates to high speed digital data networks, and more specifically refers to reducing the crosstalk between high speed data network interconnections.
2. Description of Related Art
Recently several high speed digital data services have become commercially available. These high speed digital data services are known as the ISDN (Integrated Services Digital Network) basic rate, HDSL (High Speed Digital Subscriber Loop), HDSL2 (High Speed Digital Subscriber Loop 2), ADSL (Asymmetric Digital Subscriber Loop), and T1 services. When offered by the same provider, they are typically installed in the same cable binder, and as the number of services increases, the number of cables in the cable binder increases. However, increased numbers of cables within the binder increases the crosstalk between cables, which degrades the transmission performance for each digital data service.
Each of the high speed digital data services described above typically connect two locations. The first location is typically the central office, and the other is typically a remote site. Normally, the central office site is considered the "master" and the remote site is the "slave" in terms of network synchronization. The central office is typically located at a fixed site while the remote site can be scattered over a very wide range of territory. In other words, the distance between the central office and the remote site can vary from 50 meters up to 5 kilometers depending on the location of the remote site. The cable bundle has connections distributed along the span of the cable bundle. As the number of cable pairs within the cable bundle increases, the crosstalk between pairs increases. The number of pairs within the bundle increases as the bundle gets closer to the central office, thus making the crosstalk more severe at the central office. The crosstalk interferes with proper signal transmission, thus affecting system performance.
There are two kinds of crosstalk, namely near end crosstalk (NEXT) and far end crosstalk (FEXT). In the practical network at hand where several different services are installed using different transmission methods, the NEXT is dominant over the FEXT. Therefore, methods to reduce the NEXT would have a greater effect on improving system performance.
The transmitter of the existing digital services have the nominal power required for each service. Even if the distance between the central office and the remote site is very short, the transmitter still uses the nominal transmit power and thus creates unnecessary crosstalk interference into the other pairs within the same bundle.
Although there are a few services that increase the transmit power (known as a "power burst" method) and emphasize the higher frequency content when the cable length is longer than the nominal case, the power control is not performed automatically, so additional human effort is required to maintain the service. This increases the cost of the digital service installation and maintenance. In addition, the existing power burst method is done independently, regardless of whether it is the central office or the remote site, and thus, if the power burst is performed at the central office side where the crosstalk is greater than that of the remote site, a power burst will create an even worse crosstalk environment.
It can be seen, then, that there is a need for an effective technique to reduce the crosstalk in digital data networks.